There has been known a surface light source device employing a light guide plate (see Patent Literatures 1 through 3, for example).
The light guide plate is designed and produced so as to (i) have: two end parts in its length direction, at least one of which serves as a incident surface; and two end parts in its thickness direction, which serve as an exit surface and a back surface, respectively, and (ii) direct light, emitted from a light source, incident on the incident surface, so as to cause the light to exit from a substantially entire area of the exit surface.
The light that has entered into the light guide plate is totally reflected from the exit surface and the back surface of the light guide plate, so as to exit from a substantially entire area of the exit surface.
It is desirable that the surface light source device (i) has less leakage of light from the back surface of the light guide plate, (ii) emits more light from the exit surface of the light guide plate, and (iii) has uniform luminance distribution in the light-emitting surface. In view of such a light source device, there has been conventionally known, as an advantageous technique, the use of a light guide plate which has a wedge shape as being viewed cross-sectionally in a direction orthogonal to a width direction of the light guide plate, which wedge shape has, in its length direction, (i) a root end part (an end part having a greater thickness), a surface of which serves as the incident surface, and (ii) a pointed end part. Such a light guide plate can direct more light toward its pointed end part. Therefore, a surface light source device including the light guide plate whose cross-sectional shape is a wedge shape has been widely applied to the surface light source device.
Meanwhile, a full-color LCD generally includes: a display device; and a backlight including the surface light source device. The backlight is provided on a backside of the display device. The display device includes: a liquid crystal layer; polarizers at least one of which is attached to the liquid crystal layer on a display side of the display device, and the other one(s) of which is attached to the liquid crystal layer on a backside of the display device; and color filters, provided between the liquid crystal layer and the at least one of polarizers on the display side, which allow corresponding R (red) light, G (green) light, and B (blue) light to pass through, selectively. The light guide plate of the backlight directs light emitted from the light source toward the backside of the display device. The display device controls orientation of a liquid crystal element by application of a voltage, so as to adjust an amount of light passing through each of the color filters. The display device thus carries out full-color display.
The color filters used in such a display device have high color saturation, and can develop colors in a wide color reproduction range.
However, the color filters cause an entire display to be darker, because (i) each of the color filters allows light of a corresponding predetermined primary color (wavelength) to pass through while causing light of other colors to be less intense, and (ii) this reduces the amount of light which has passed through the color filters to approximately one-third of the amount of light emitted from the light source.
In view of the problem, there has been known a technique of using dichroic filters in place of the color filters such that the dichroic filters are provided on a surface (the exit surface of the light guide plate of the surface light source device) of the backlight (see Patent Literature 4). Each of the dichroic filters reflects visible light having a wavelength other than a specific wavelength so as to selectively allow visible light having the specific wavelength to pass through.